Re: Converting I and Q to real signal

Tim Wescott wrote:
micdahl wrote:
How to interpret these signals in terms of your system depends on just
how
your system is massaging things. If it is just sending out a ping and
giving you the nearly-raw demodulated result, then the phase of a
reflection will depend on the distance of the reflecting surface from
the
transducer. If that surface is moving then the phase will rotate, so
you
would essentially want to build a FM detector on your returns; then the

output of the detector would be proportional to the velocity of the reflecting surface.


*snip*
You sort of lost me on the FM detector. I aquire the signal already
split

into separate I and Q data. Can I just assume the velocity is
proportional

to the angle change in the "phasor" the I/Q data make in the complex
plane?

(assuming that this angle represents the phase difference between the

recieved signal the tranducer gets from the object and a reference
signal,

therefore denoting relative position).
Yes. If you think like a radio guy you'd do that by implementing an FM
detector -- we're talking about the same thing, just in a different
language. You may find useful information for your problem by looking
up
I-Q FM demodulation. It sounds like you now have a sufficient grasp of
the issues, however.

Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html


Thanks for all the advice about FM demodulation, this problem now looks
deceptivly easy. Judging on what i've read about FM demodulation, I just
have to run a software loop with:

=[arctan(Qn/In)-arctan(Qn-1/In-1)]/delta t
or = I*dQ/dt - Q*dI/dt

Which should give me the frequencies of my vibration, even if the
amplitudes are not to scale.
Unfortunatly when reading about FM demodulation I got confused about
whether or not I needed to filter and/or decimate my data. If my sampling
frequency of I and Q (1563hz) is just over double the highest frequency in
my signal I care about (~700Hz) can I just call it a day and not worry
about it? If it does produce noise, would it be in the 0-700Hz range that
I care about?
Thanks!

_____________________________________
Do you know a company who employs DSP engineers? Is it already listed at http://dsprelated.com/employers.php ?

You might like to filter your data to get rid of aliases, but the sampling is happening "out there" so it's inaccessible. I.e. the sampling is defined by the time that the pulse is generated; this means that all information between the last pulse and the next is already lost before you've even collected one byte of data from the next pulse. Thus, there's no place to put a filter.

So you'll have to live with the aliases and hope they don't bite you, or connive to increase the machine's sampling rate.

Having both I and Q signals effectively doubles the sample rate. Sampling more than twice the highest frequency in the signal refers to the total number of samples in a given time. In this case, both I and Q.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
.

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